5.4 Surfaces

To predict surface reconstructions, you have to:

Specify 200 or 201 : calculationType.
Provide a file containing substrate in VASP5 POSCAR format, as shown on Fig. 12. This can be done using online utility: https://uspex-team.org/online_utilities/substrate/ (after you get your slab model, you need to put it in your folder and run USPEX one time — it will generate for you the "final" POSCAR_SUBSTRATE file at stop. With this file you can now run your USPEX calculations).
Do not forget to specify vacuumSize which should be sufficently large to prevent interaction between periodic images of slabs.
Specify the following parameters:

$\includegraphics[scale=0.5]{pic/Surface.png}$

Figure 12: Surface model used in USPEX. Note that POSCAR_SUBSTRATE shall exactly represent the geometrical information of its bulk crystal without vacuum. If the input has large vacuum region, the program will automatically delete it and provide a new file called POSCAR_SUBSTRATE_NEW, and this file has to be used in the calculation (renamed as POSCAR_SUBSTRATE).

% symmetries
2-17
% endSymmetries

Note: If the symmetries tag is present, USPEX will try to generate surface structures using plane groups.

$\triangleright$ variable thicknessS

Meaning: Thickness of surface region. Adatoms are allowed only in this region.

Default: 2.0

Format:

3.5 : thicknessS

$\triangleright$ variable thicknessB

Meaning: Thickness of buffer region in substrate. This region is part of POSCAR_SUBSTRATE, and is allowed to relax.

Default: 3.0

Format:

3.0 : thicknessB

$\triangleright$ variable reconstruct

Meaning: Maximum multiplications of the surface cell, to allow for complex reconstructions.

Default: 1

Format:

1 : reconstruct

USPEX is capable of predicting surface reconstructions with variable number of surface atoms. In this case, stable surface reconstructions are dictated by chemical potentials ²⁸. A typical set of input is the following:

******************************************
*      TYPE OF RUN AND SYSTEM            *
******************************************
USPEX  : calculationMethod (USPEX, VCNEB, META)
201    : calculationType (dimension: 0-3; molecule: 0/1; varcomp: 0/1)

% atomType
Si O
% EndAtomType

% numSpecies
2 4
% EndNumSpecies

Here we specify the maximum number of surface atoms in a $1 \times 1$ cell.

******************************************
*               SURFACES                 *
******************************************
% symmetries
2-17
% endSymmetries

% StoichiometryStart
1 2
% StoichiometryEnd

This defines the stoichiometry of the bulk.

-23.7563 : E_AB (DFT energy of AmBn compound, in eV per formula unit)
-5.4254  : Mu_A (DFT energy of elemental A, in eV/atom)
-4.9300  : Mu_B (DFT energy of elemental B, in eV/atom)

3.5      : thicknessS (thickness of surface region, 2 A by default )
3.0      : thicknessB (thickness of buffer region in substrate, 3 A by default)
4        : reconstruct (maximum multiplications of cell)

At the moment, USPEX supports variable-composition calculation for the following cases:

Reconstructions of elemental surfaces (such as C@diamond(100) surface).
Reconstructions of surfaces at binary compounds (such as GaN(0001) surface).
Reconstructions involving foreign species on elemental surfaces (such as PdO@Pd(100) surface).